1. Field of the Invention
The invention concerns vessels for lifting heavy loads offshore, and particularly vessels for removing offshore platforms with steel jackets, after the platform deck is removed for instance by means of another lifting vessel. A method for using such vessels is also enclosed.
2. Description of Related Art
As offshore oilfields throughout the world are being depleted, and when further exploitation no longer is profitable, platforms will have to be decommissioned or shut down. It is required from various authorities and international cooperation treaties (OSPAR Convention decision 3/98 and the Guidelines of the Maritime Organization IMO) that all platforms that have been taken out of service, and that can be removed, shall be brought onshore for removal.
On the Ekofisk field alone, soon 15 platforms will be decommissioned and removed. Most of these platforms include a steel jacket on the seabed with a platform deck on top.
The steel jackets are distinguished by being of a truss work design where the weight of the platform is transferred through the legs of the jacket down to the seabed, and from there further down through poles. These designs or constructions are relatively weak in a transversal direction and do not have sufficient strength to transfer its own weight without many supporting points, localized on the joints or nodes between the legs and the transversal supports. Typical for these constructions are 4 to 12 legs with a weight of up to 20,000 tons.
A conventional method for the installation of such jackets is that these are laid horizontally on a barge carrier and thereafter either lifted or pushed into the sea, rotated vertically, and lowered down to the seabed by means of a crane vessel rotating the jacket vertically before being lowered to the seabed. If the jacket is too heavy, the filling of buoyancy tanks is used as a supplementary aid. During removal, this sequence can be reversed. However, it is a problem that such jackets frequently are difficult to turn, and this means that they often have to be cut into smaller parts for transport to the shore. The conventional lifting vessels have a limited lifting capacity, and the use of such requires a lot of under water work. This work can include installation of new lifting lugs, cutting off legs and crossbeams and possibly the use of buoyancy tanks either alone or in combination with a lifting vessel. Transferring the jacket to a barge for transport to the shore also requires calm weather conditions with low wave heights, something that possibly lengthens these operations and thereby makes them more financially demanding.
Other vessels of various designs have been suggested to simplify these operations.
Norwegian Patent Application 1996 5439 shows a transporting device for the installation and removal of platform underframes or jackets. The application shows that the transporting device is an elongated construction with floating elements that can be ballasted such that the transporting device takes a substantially vertical position in the sea. The transporting device is then led towards the jacket that is to be lifted, and is secured to a sliding saddle on the transporting device. The transporting device is then deballasted such that it is lifted along with the jacket to a substantially horizontal position.
U.S. Pat. No. 4,651,667 shows a pivoted barge for the towing and launching of offshore constructions. A joint pivots as the construction, that is to be launched, is displaced along with its center of gravity and results in a pivoting movement of the joint.
The building of new vessels is however fairly expensive and necessitates many marine systems in addition to fabrication of steel material. At the same time there are a large number of vessels tied up waiting for work. These may already be equipped with systems for safety, ballasting, mooring, towing and propulsion in addition to accommodation, cantinas and large deck areas. These systems have also been tested, certified and used. By attaching a lifting structure to an existing vessel, the expenses for a lifting or decommissioning vessel is considerably reduced. It is expected that an amount of between 5,000 and 10,000 tons of new steel can be saved. Such existing vessels can be fairly inexpensive, for instance due to the present situation in the North Sea. The integration between the existing and the new constructions can be kept to a minimum to control the expenses. Making a lifting construction suited for connection to different vessels makes it possible to keep the day rates to a minimum. By integrating these additional services in the lifting vessel, the need for additional vessels during the preparation offshore will be limited.